1. Field of the Invention
This invention relates generally to gyroscopic apparatus and more particularly to a device for precessing a fluid sphere gyrocompass and compensating for undesirable drift and error characteristics.
2. Description of the Prior Art
The use of a gyroscope as a north-seeking element of a gyrocompass is well known in the art. Basically, the north finding operation of the conventional gyrocompass derives from the phenomenon that when the spin axis of a gyroscope which was originally pointed north is not accurately pointed along the meridian, the gyroscope will rotate about a tilt axis horizontal (relative to the earth) and normal to the spin axis at a rate equal to the horizontal earth rate component of angular velocity along that axis. This rotation results in a tilt of the spin axis away from the horizontal, which can be sensed and used to apply a restoring torque to redirect the spin axis to true north. Thus, to redirect the spin axis in the north-seeking direction, if the gyroscope spin axis varies from its nornal northerly position, a restoring couple may be applied to the tilt axis, normal to the spin axis and the azimuth plane. This precesses the gyro spin axis toward the northerly direction. Direction may then be read from an azimuth card, using the spin axis to indicate north.
Since the north seeking operation is basically oscillatory in simple harmonic motion, means must be provided to damp the oscillation. This has been done by applying an out-of-phase torque to either the horizontal or vertical axis.
It is also well known that a gyrocompass in a two-axis system, gimballed with the tilt axis horizontal and the azimuth axis normal to the spin axis and tilt axis, will not show a correct heading in a rolling or pitching vehicle unless the vehicle is on a cardinal course (north, south, east, or west), due to the so-called intercardinal rolling error which subjects a gyrocompass to a net vertical-axis torque. The influence of symmetrical rolling does not average out on an intercardinal course, and the error may be of the order of 5.degree. for .+-.10.degree. roll with 8-second periods. Since the pitch angle is generally a smaller source of error, compensation must be provided at least to correct for excessive intercardinal rolling error. This is commonly corrected by providing a delay to the error signal such as by using a ballistic pendulum with a viscous liquid for damping, or electrical delay applied to the signal from an acceleromter. The delay results in a negligible net vertical-axis torque over the roll cycle.
It is further known to those skilled in the art that in order for a gyrocompass to remain continuously pointing north in the presence of vehicular velocity and acceleration, and in a changing latitude, compensation for these dynamic effects, which otherwise result in north pointing error, must be provided. For example, corrections must be applied for the well-known northerly velocity error which results from the settling of the gyro spin axis along the normal to the total horizontal angular velocity vector (vector sum of the horizontal component of the earth angular speed with ship's angular speed about the center of the earth). It is a function of the ship's speed, course, and latitude, and may be corrected either by application of a suitable vertical axis compensating torque to the gyro, or by periodically offsetting the readout angle of the azimuth card. Commonly it is corrected by applying a vertical axis torque proportional to the northerly component of vehicle velocity. This precesses the gyro about the tilt axis at the exact rate required to keep the spin axis tangent to the surface of the earth at all times as the vehicle proceeds in a given direction, and the gyroscope remains centered on the meridian. Preferably, an acceleration correction will also be applied to preclude oscillation about the meridan when the heading or speed is changed.
In addition to the dynamic corrections described above, which are independent of the construction of the gyroscope, corrections are also required for bias and acceleration drift peculiar to the structure of the gyroscope and its mounting arrangement. This includes acceleration sensitive drifts which may occur due to unbalance of the spin and input axes, structural compliance, aniso-elasticity of the mounting members, and bias drifts such as thermal torques, frictional torques due to normal forces on bearings, and electromagnetic and electrostatic torques from stray fields, electrical pick-offs, and torque motors, etc. This list of bias and acceleration sensitive drift responses is by no means comprehensive but is merely illustrative of the potential errors inherent in the gyroscope.
The prior art has described an improved type of gyroscope employing a fluid mass housed in a rotary container, as disclosed by W. G. Wing in U.S. Pat. No. 3,058,359, entitled Fluid Rotor Gyroscopic Apparatus, issued Oct. 16, 1962, and assigned to the assignee of the present invention. The fluid sphere rotor gyroscope offers notable advantages over the conventional form of solid rotor type of gyroscope in that the need for compensating mechanical adjustments for bias and acceleration sensitive drift is minimized. As a result of these characteristics, the fluid rotor form of gyrocompass can provide improved reliability over the conventional gyrocompass with a minimum of assembly, test and calibration time required. In a gyroscope of this type, a cavity or rotor containing a mass of fluid is rotated and thereby spins the fluid body. The fluid body then tends to maintain a fixed orientation in space with changes in the orientation of the container, thereby causing the spin axis of the fluid to deviate from the spin axis of the container. By means of pressure sensitive transducers, mounted within the container, an output signal is provided when the spin axis of the fluid and that of the container are not coincident due to external rotation of the gyroscope. The output signal is representative of the angular difference between the spin axes and is measured as a change in differential pressure in a manner more fully explained in the aforementioned patent.
However, it has been found that when there is an axial temperature difference existing in the fluid which forms the sensitive element, such as may be introduced by dissipation of electrical energy by the rotor driving source, the output signal is in error by what appears as a drift when an acceleration is present. The source of the problem is the axial temperature gradient which causes a density variation in the fluid and a consequent unbalance in the gyroscope. In U.S. Pat. No. 3,200,653, issued Aug. 17, 1965 also to W. G. Wing, and also assigned to the assignee of the present invention, an improved form of the fluid sphere rotor gyroscope uses transverse members of high thermal conductivity along the spin axis to reduce steady state errors due to such thermal gradients. However, that invention does not provide for dynamic corrections, such as to compensate for drifts due to acceleration.
Thus it is seen that the prior art requires numerous corrections to compensate for undesirable bias and acceleration sensitive drifts, and that such corrections may require precise and time consuming adjustments in a conventional rotor gyroscope. The present invention provides a simplified compensation means for the above drift sources which makes controlled use of the acceleration sensitive drift-to-temperature gradient along the spin axis of a fluid sphere rotor gyroscope, which has heretofore disadvantageously been a source of drift and error. The same effect is also utilized to provide a novel way to precess the gyroscope about the vertical axis in a controlled manner to compensate for the vertical component of earth's rate of rotation and to precess the gyroscope spin axis, as required, toward north.